1. Field of the Invention
The invention relates to a control system for a vehicle which performs shift control of an automatic transmission, and particularly relates to a technology of performing engine torque down control through fuel cut control during an inertia phase of a power-on upshift.
2. Description of Related Art
In a vehicle including an engine having a plurality of cylinders, and an automatic transmission that transmits power from the engine toward driving wheels, it is well known to perform engine torque down control for cancelling inertia torque during an inertia phase of an upshift while the accelerator pedal is being depressed (namely, a power-on upshift) so as to suppress shift shocks. For example, a shift control system for an automatic transmission as described in Japanese Patent Application Publication No. 10-59022 (JP 10-59022 A) is used in this type of vehicle. In JP 10-59022 A, as a method of increasing the amount of torque reduction during the inertia phase as a target gradient of the input shaft speed of the transmission is larger (namely, as the shift speed is higher), it is proposed to reduce torque by cutting off fuel supply to at least one of the cylinders of the engine (performing fuel cut control), instead of retarding the ignition timing of the engine, when the shift speed is high. In recent years, the automatic transmission may be desired or required to be sharply (or quickly) shifted up or down in some cases, and the above-described method is useful in such cases.
In the meantime, under the fuel cut control performed on a part of the cylinders (which will be called “partial fuel cut control”), intake and exhaust valves of the engine and pistons received in the cylinders keep operating or moving, unlike cylinder de-activation control; therefore, under the partial fuel cut control in the case of a power-on upshift, in particular, air is fed into a catalyst provided in an exhaust pipe of the engine. As a result, exhaust gas that flows into the catalyst has a lean air-fuel ratio (large A/F), and it may become difficult for the catalyst to convert or remove NOx, in a system that reduces nitrogen oxides (NOx) utilizing the engine operation at the stoichiometric air-fuel ratio (stoichiometric A/F) in combination with the three-way catalyst. In another point of view, driving force that meets a requirement needs to be produced in a power-on upshift. When the partial fuel cut control is performed, the torque may be excessively reduced since the amount of torque reduction can only be increased in steps on a cylinder basis. In view of this possibility, it is considered to increase the throttle opening so as to make up (compensate) for at least a part of the torque reduction amount, using one or more cylinders to which the fuel continues to be supplied. As a result, a larger amount of air is fed into the catalyst via the cylinders from which the fuel is cut off, and it may become difficult for the catalyst to convert or remove NOx. The above-described problem has not been publicly known, and it has not been proposed to curb deterioration of emissions when the partial fuel cut control is performed.